Upon measuring the pressure of aggressive fluids, a manometer is used which cooperates with a diaphragm device so as to be protected from these aggressive fluids. The diaphragm device constitutes a separate unit which is connected to the aggressive fluids, on the one hand, and to the manometer, on the other hand, and incorporates a diaphragm separating the manometer from the fluid. Consequently, the pressure exerted by the aggressive fluid on one side of the diaphragm is transmitted to the manometer by means of an intermediate neutral fluid provided at the other side of the diaphragm.
Since such a pressure measuring apparatus employs a separate diaphragm device, the entire unit i.e. diaphragm device and manometer is of considerable size, thus requiring a considerable amount of space. Moreover, the neutral transfer fluid must be filled into the diaphragm device without any gas occlusions or vacuum occlusions. During connection of the diaphragm device to the manometer, these occlusions could, however, not be entirely prevented. A further disadvantage of such a prior art apparatus resides in the complexity of the pipework in case of connecting a vertical line.